(a) Field of the Invention
This invention relates to needled felts used in the press section of a paper making machine and is particularly directed to the provision of an improved base fabric for a wet felt having a batt of fibers needled thereto.
(b) Description of Prior Art
In the continuous manufacture of paper from a pulp suspension, the paper machine comprises essentially a forming section, a press section and a dryer section. In the forming section a thin suspension of fibers and fillers, containing generally about 99.5% water, is flowed from a headbox slice at the upstream end onto the surface of a moving endless screen belt or forming fabric which is made of woven metal or plastic filaments. The forming fabric passes over various devices which withdraw some of the water from the pulp stock, leaving on the fabric a thin self-supporting web of matted fibers containing about 75 to 80 percent water.
The web of fibers is lifted off the forming fabric at the downstream end of the forming section and is transferred to a press section where it is deposited on a series of endless belts of relatively thick, permeable, water-absorbing felt and is passed on these felts between one or more sets of press rolls where some of the water remaining in the web of paper is transferred to the felts by pressure.
After emerging from the press section and containing about 60 to 65% water, the paper web is then transferred to a dryer section where it runs in serpentine fashion over a number of steam heated rolls and the remaining moisture in the web is driven off by evaporation.
The evaporation of the 60% of moisture remaining in the paper web is a costly process as it requires a considerable amount of energy in the form of steam. It will be appreciated that if more water is removed from the web in the press section, less steam will be required in the dryer section. For example, in a machine producing 600 tons of heavy paper a day, a reduction in moisture content of only 2 percent in the web delivered to the dryer section will result in a saving of 216,000 pounds of steam each day. In terms of money this will amount to a daily saving of about $540.
Water removal in the press section is effected by the use of a smooth surfaced, perhaps rubber covered, top roll bearing under pressure against a grooved, perforated or mesh covered bottom roll which provides receptacles for water expressed from the paper web and felt as they pass between the rolls. The felt, which is compressible and resilient, acts as an intermediary between the water receptacles and the paper web. A generally accepted theory is that as the paper web and felt approach the maximum nip between the press rolls, water is squeezed from the paper to the felt. At the maximum nip the compressed felt has reached the saturation point resulting in the flow of water from the felt into the receptacles in the roll under the felt. After passing the maximum nip the resilient felt expands, the paper is further compressed until it reaches a state of maximum dryness, then, as air and water from the receptacles in the roll enter the expanding felt, a negative pressure is created in both paper and felt and, while the felt will retain most of the water some will be re-absorbed by the paper. Water is removed from the felt by passing it over a suction box and residual water is removed from the bottom press roll by centrifugal action and/or by a doctor.
An ideal press felt should provide perfectly uniform pressure distribution, the lowest possible resistance to flow of water through the felt, perpendicularly and in the machine direction, and compressibility and resilience so that it will have full elastic recovery after passing through the press nip in order to minimize re-wetting of the paper web. In addition to these water extraction efficiency factors, the ideal press felt should have a smooth, non-marking paper contacting surface and sufficient strength and stability to prevent length and width variations and wrinkling during operation.
A conventional type of felt that most nearly approaches this ideal consists of an openmesh base fabric woven with synthetic monofilaments or multifilaments, or a combination of these, to which is needled one or more batts of staple fibers. The base fabric should have low compressibility to maintain integrity and may be single layer or socalled double or multi-layer fabric having two or more layers of weft. The base fabric is generally woven endless so that when the felt is installed on the machine the weft is continuous and extends in the running direction; the warp extending in the cross-machine direction.
Examples of prior art press felts are described and illustrated in U.S. Pat. Nos. 3,214,327, Wicker et al, and 4,107,367, Fekete.
When the web travels through the dryer section, it is also carried by an endless belt. One such endless belt is taught and illustrated in U.S. Pat. No. 4,290,209, Buchanan et al. In the dryer fabric illustrated in the '209 patent, at least the warp strands are flattened in cross-section.